mirror of https://gerrit.osmocom.org/libosmocore
Vadim Yanitskiy
7790459c8e
As was demonstrated in [1], there is a TCH/AHS specific problem in libosmocoding causing unexpected BER ~50% in decoded AHS_SID_UPDATE frames. The reason is that A[H]S_SID_UPDATE employs quite tricky interleaving algorithm, which is different from the algorithm used by normal TCH/AHS speech frames or A[F]S_SID_UPDATE frames. An AHS_SID_UPDATE frame consists of two halves (228 bits each): +---------+--------------------|---------+--------------------+ | in-band | SID marker | in-band | coded data | +---------+--------------------|---------+--------------------+ | 16 bits | 212 bits | 16 bits | 212 bits | The first half contains coded in-band signalling data (16 bits) and the identification marker (212 bits), which allows to detect that it's an AHS_SID_UPDATE. This half is carried by even bits of the first two bursts and odd bits of the last two bursts. The other half also contains the in-band data (16 bits), while the remaining 212 bits contain encoded SID_UPDATE (212 bits). This half is carried by even bits of the last two bursts and odd bits of the first two bursts. Current implementation does not use odd bits of the first two bursts at all, so buffer cB[] in gsm0503_tch_ahs_decode_dtx() contains only 114 out of 228 bits. This patch changes the logic, so that gsm0503_tch_ahs_decode_dtx() would not split AHS_SID_UPDATE onto two frames anymore like its TCH/AFS equivalent does, but attempt to deinterleave the second half and attempt to decode the payload immediately. Change-Id: I8686d895e96fa0e606c1898b6574cc80a8f46983 Related: [1] I434157e2091a306c039123cea08d84bd8533c937 Related: SYS#5853 |
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| .. | ||
| Makefile.am | ||
| gsm0503_amr_dtx.c | ||
| gsm0503_coding.c | ||
| gsm0503_interleaving.c | ||
| gsm0503_mapping.c | ||
| gsm0503_parity.c | ||
| gsm0503_tables.c | ||
| libosmocoding.map | ||